Deleting a field or section is easy: simply click on the bin/trash symbol on the left-hand side, as shown in the image below:
Deleting a section will automatically delete all of the fields within the section.
The impact of deleting a field
Deleting a field can have a major impact on your form/document if that field is being used in any logic throughout the system or as a merge field in your document. If so, the platform will warn you about this before confirming the deleting (see here for more details).
Data for deleted fields
Deleting a field will not remove the data that was entered for that field in previous submissions, so the veracity of those previous submissions will be retained even after the field is deleted.